In the case of data services, a mandatory requirement of a Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (UTRAN) is to provide dedicated channels for transmission of user terminal packet data on the air interface (i.e. by radio between a base station and mobile user terminal terminals). It is given that there will be more user terminals than dedicated channel resources and so a method is required to share the available resources. One method is use of an inactivity timer. The inactivity timer monitors activity of a user terminal and deallocates dedicated resources when the period of inactivity exceeds the threshold for the inactivity timer. This behaviour gives rise to multiplexing gain. The user terminal nevertheless retains a logical connection to the Core Network and any data server to which they are connected.
Effective utilisation of dedicated channels can only be achieved when a large percentage of the time the dedicated channel is being used to transfer data. This is often not the case when small files are being transferred and the transport layer is using congestion avoidance algorithms, for example Traffic Control Protocol (TCP) with slow start. In this case, round trip delays within the network dominate the maximum transfer rate. Particularly inefficient use of the air interface and dedicated channels is observed when short periodic ‘keep alive’ messages are being sent between client and server, as occurs for some application types. Such traffic involves the establishment of a dedicated channel, the transmission of a small amount of data, and the activity timer to expire before the channel can be released. The inactivity timer duration exacerbates the problem of user data being sent for only a small proportion of the time (i.e. a low duty cycle) when using dedicated channels.
A potential solution to this problem is to use signalling channels to transmit data. Termed the Forward Access CHannel (FACH) and Reverse Access CHannel (RACH), these common channels possess the advantage of carrying short, bursty (i.e. low duty cycle) traffic more efficiently over the air interface. This has the closely associated benefit of good (i.e. low) signalling behaviour, as dedicated channels are not regularly being set up and torn down. Another benefit is low channelisation code usage, the maximum number of codes usable at any one time is a significant limitation for system performance in high data rate cases.
Currently the parameters used to decide when a user terminal terminal is to transition to and from the state where data is sent over the signalling channels is based on the prevailing data activity for that user terminal terminal (i.e. data rate and buffer occupancy).
The transport of user data, together with signalling for air interface management, over the FACH/RACH channels results in a significant increase in the time for which the FACH/RACH channels are in use (i.e. FACH/RACH duty cycles). In systems where the FACH channel is not power controlled, the transmit power (from the base station) for the FACH needs to be at such a level that it reaches the edge of the cell, regardless of the location of the mobile user terminal terminals. In addition, the proportion of the time for which the RACH channel is in use (i.e. the RACH duty cycle) will naturally also increase resulting in a increase in the uplink loading. The RACH channel also has no fast power control and so with fast fading can reduce the performance of the base station receiver.
In addition, for Acknowledged Mode services, data transmission on a FACH/RACH channel pair involves large number of acknowledgements being transmitted as part of the Radio Link Control (RLC) behaviour. The RLC is a so-called layer 2 protocol and provides reliable data transfer. Depending on a user terminal terminal's traffic profile there is a certain amount of interference in both uplink and the downlink directions (i.e. to and from the base station) due to the additional acknowledgement messages.
The FACH/RACH solution is intended for ‘bursty’, low volume data transfer, while the dedicated channel is intended for high-volume constant data transfer. When a user terminal is sending or receiving data on the FACH/RACH channels, it is intended that as data volume increases, the user terminal moves from the FACH/RACH channels to the dedicated channel state. Metrics (i.e. measurements are made and used to determine when a user terminal terminal should transition from one state to another, specifically monitoring data rate and buffer occupancy for a given user terminal terminal.